This invention generally relates to a coating composition for UV protection. More particularly, the invention relates to a UV protective coating composition for protection in optoelectronic devices. The invention also relates to optoelectronic devices using such coatings.
Materials like thermoplastic resins often possess an attractive set of mechanical and physical properties, such as high heat resistance, impact resistance, dimensional stability, high ductility, and optical clarity. However, these materials are usually susceptible to photodegradation by ultraviolet (UV) light. This type of degradation typically leads to erosion and yellowing of the polymer surface, e.g., in the case of polymer substrates. Efforts to alleviate these problems have been undertaken in the past. Optically transparent coatings are employed to protect polymeric substrates and other substrates from the effects of the weather. Highly weatherable coatings are required for many applications such as architectural glazing, automotive glazing, and solar energy (e.g., protective coatings for photovoltaic modules). These applications require maintenance of properties for a long duration of time, for example, at least 15 years for automotive use and sometimes, more than 25 years for solar and architectural applications. Most of these coatings employ organic UV absorbers to give good weathering properties. As an example, coating compositions that include ultraviolet light absorbers have been applied onto thermoplastic substrates and then cured.
The UV absorbing agents themselves sometimes decompose to some degree, upon exposure to UV light. UV absorbers degrade at various rates, and the rate can be dependent on the polymer matrix in which it resides. The decomposition results in decreasing the UV blocking capability, and thereby, exposing the underlying substrate to the degrading effects of UV light, abrasion, and the like. Thus, the purpose of the coating in protecting the substrate is defeated to some extent. A way to mitigate UV absorber loss is to increase the UV absorber loading and/or increase the thickness of the coating. However, there are practical limitations to this approach since high additive loadings and/or thick coatings can adversely affect the physical properties of the coating and/or substrate as well as increase costs. Therefore, to make very highly weatherable coatings, one needs very stable UV absorbers.
Certain triazine UV absorbers have high molar absorbtivity and excellent photostability but are poorly compatible with desirable coating matrices. Photostability is defined as loss of absorption per MJm−2 nm−1 (measured at 340 nm) of xenon arc exposure. However, certain dibenzoylresorcinols have excellent photostability and good compatibility with coating matrices, but have relatively low absorbtivity.
Therefore, there is a need for new coating compositions, which are effective in providing UV protection for a variety of different types of substrates. The compositions should exhibit a high degree of UV stability, i.e., photostability, while still being effective in protecting the substrate. Additionally, the compositions should have high absorbtivity while minimizing coating thickness and UV absorber loading. In addition, the compositions should be amenable to various forms of use, e.g., they should be capable of being easily applied to a surface being protected, without crystallizing out. The coating compositions should also not interfere with any of the other properties possessed by the substrate, such as transparency. Furthermore, the compositions should be relatively easy to manufacture, and their use should not involve an excessive increase in the cost.